Eczema is caused by a combination of genetic predisposition, immune system overactivity, and a weakened skin barrier, not by any single factor. An estimated 129 million people worldwide have atopic dermatitis, the most common form of eczema, and the condition results from these internal factors interacting with environmental triggers like dry air, irritants, and certain microbes. Understanding what drives eczema helps explain why it behaves differently from person to person and why flares seem to come and go unpredictably.
A Faulty Skin Barrier Starts the Problem
Healthy skin works like a brick wall. Skin cells are the bricks, and a mixture of natural fats (called ceramides and other lipids) acts as the mortar, sealing moisture in and keeping irritants out. In eczema-prone skin, this mortar is defective. Research published in the Journal of Lipid Research found that people with eczema have drastically higher levels of abnormally short-chain ceramides and reduced levels of the longer-chain ceramides that are essential for a tight seal. These changes in fat composition correlate strongly with increased water loss through the skin, which is the measurable sign of a compromised barrier.
When the barrier leaks, two things happen simultaneously. Moisture escapes, leaving the skin dry and prone to cracking. And allergens, bacteria, and chemical irritants slip through gaps that wouldn’t exist in intact skin. This sets off the immune response that produces the redness, swelling, and intense itch characteristic of eczema. The barrier problem isn’t just a symptom of inflammation. It’s one of the root causes.
Genetics: The Filaggrin Connection
The single strongest genetic risk factor for eczema is a mutation in the gene that produces a protein called filaggrin. This protein is essential for building the outermost layer of skin and maintaining its protective function. When the gene is mutated, the body produces less filaggrin (or none at all), and the skin barrier forms improperly from the start. Up to half of all people with atopic dermatitis carry a filaggrin gene mutation.
That said, genetics isn’t destiny. Not everyone with a filaggrin mutation develops eczema, and many people with eczema have perfectly normal filaggrin genes. Researchers have identified dozens of other genes involved in skin structure and immune regulation that each contribute a small amount of risk. Eczema runs in families partly because of these inherited vulnerabilities, but it takes environmental exposure to push susceptible skin into active disease.
An Immune System Tilted Toward Inflammation
In eczema, the immune system overreacts to substances that wouldn’t bother most people. The specific pattern involves a branch of immunity called the type 2 inflammatory response. When triggered, certain immune cells flood the skin with inflammatory signaling molecules, particularly two that play outsized roles in driving eczema symptoms. These molecules recruit other immune cells, including those responsible for allergic reactions, and they stimulate the production of antibodies associated with allergies. The result is a self-reinforcing loop: inflammation damages the skin barrier further, which lets in more irritants, which triggers more inflammation.
This immune pattern also explains why eczema so often appears alongside other allergic conditions. About 43% of children with eczema go on to develop asthma, and 45% develop allergic rhinitis (hay fever). Among children with severe eczema, roughly 70% eventually develop asthma. This progression from eczema in infancy to respiratory allergies later in childhood is sometimes called the “atopic march,” and it reflects the same underlying immune tendency expressing itself in different organs over time.
Bacteria on the Skin Play a Larger Role Than Expected
Everyone’s skin hosts a community of microbes, but in eczema the balance shifts dramatically. A bacterium called Staphylococcus aureus colonizes eczema-prone skin at far higher rates than healthy skin, and its overgrowth reduces the diversity of other, beneficial microbes. S. aureus doesn’t just passively sit on damaged skin. It actively worsens the condition by producing toxins, triggering inflammatory responses, and further weakening the skin barrier.
The relationship works both ways. Genetic mutations like those affecting filaggrin create a more hospitable environment for S. aureus by raising the skin’s pH and reducing the natural antimicrobial compounds the skin normally produces. The bacteria then amplify the inflammation already underway, which is why eczema flares often involve a visible worsening that goes beyond simple dryness. Colonized patches tend to be more red, more swollen, and more intensely itchy.
Common Triggers That Set Off Flares
Having the genetic and immune predisposition for eczema doesn’t mean constant symptoms. Most people experience a pattern of flares and remissions, and specific environmental triggers are usually responsible for tipping the balance. The most well-established triggers include:
- Low humidity and central heating: Dry air pulls moisture from already-compromised skin. The National Eczema Society recommends keeping indoor humidity between 30% and 60% and room temperature around 18°C (64°F). Overheated homes in winter are a common and underappreciated trigger.
- Irritants: Soaps, detergents, fragrances, and certain fabrics (especially wool) can provoke flares by directly irritating the skin or stripping away its limited protective oils.
- Sweating and overheating: Physical warmth dilates blood vessels in the skin and can intensify itching, turning minor irritation into a full flare.
- Stress: Psychological stress amplifies the type 2 immune response, which is why many people notice their eczema worsens during high-pressure periods.
- Allergens: Dust mites, pet dander, pollen, and mold can trigger immune reactions in the skin, particularly when the barrier is already compromised enough to allow these particles in.
Food Allergies: A Trigger, Not a Cause
One of the most common misconceptions about eczema is that food allergies cause it. The relationship actually runs in the opposite direction. Growing evidence supports that eczema comes first, and a damaged skin barrier then allows food proteins to enter the body through the skin, sensitizing the immune system and eventually leading to food allergies. In other words, eczema predisposes children to food allergies, not the other way around.
That said, once a food allergy develops, eating the offending food can worsen eczema symptoms. This creates a confusing picture for parents who notice their child’s skin flares after eating certain foods and understandably assume the food is the root problem. Eliminating foods without proper allergy testing is rarely helpful and can lead to unnecessary dietary restrictions, particularly in children who need broad nutrition for growth.
Other Forms of Eczema Have Different Causes
Atopic dermatitis is the most common type, but “eczema” is actually an umbrella term covering several related conditions. Seborrheic dermatitis, which causes flaky, scaly patches on the scalp, face, and chest, is driven by an overgrowth of a yeast called Malassezia that naturally lives on oily areas of skin. When Malassezia proliferates, it produces toxic byproducts and reactive oxygen species that irritate the skin and trigger inflammation. This is why antifungal treatments work for seborrheic dermatitis but do nothing for atopic dermatitis.
Contact dermatitis, another common type, is caused by direct exposure to a specific irritant or allergen, such as nickel, latex, or poison ivy. Unlike atopic dermatitis, it doesn’t require a genetic predisposition and can develop in anyone with enough exposure. Dyshidrotic eczema, which produces small, intensely itchy blisters on the hands and feet, has triggers that overlap with atopic dermatitis but tends to be more closely linked to stress, sweating, and contact with metals like nickel or cobalt.
Why Eczema Often Starts in Childhood
Most atopic dermatitis appears before age five, and many cases begin in the first year of life. This timing reflects the fact that infants’ skin barriers are still maturing, and the immune system is still learning to distinguish between harmless and harmful substances. In a child with genetic susceptibility, this developmental window creates a period of heightened vulnerability. By one study’s findings, 82 out of 94 children with early eczema saw improvement by age seven, though many went on to develop other allergic conditions instead. By age three, nearly two-thirds of children with confirmed eczema already showed signs of asthma, allergic rhinitis, or both.
Adults can develop eczema for the first time too, though it’s less common. Adult-onset eczema may involve different immune pathways and tends to appear on the hands, eyelids, and flexural areas. The same core triad of barrier dysfunction, immune overactivity, and environmental triggers applies, but the balance of these factors can look different than it does in childhood-onset disease.